Ethylene is produced in the petrochemical industry by steam cracking. In this process, gaseous or light liquid hydrocarbons are heated to 750-950° C., inducing numerous free radical reactions followed by immediate quench to stop these reactions. This process converts large hydrocarbons into smaller ones and introduces unsaturation.
The production of ethylene comprises a primary compression of the cracked gas (3 stages or phases of compression) followed by a secondary compression (1 or 2 stages or phases). Hydrogen sulfide and carbon dioxide are removed usually during secondary compression.
For the primary and secondary compression turbo-compressor trains may be used, of the type comprising five machines mechanically coupled namely: a steam turbine, a gear box, a first group of low pressure compressors, and a second group of high pressure compressor. The gear box is a step-up type of gear, for example, increasing the rotation speed to 5000 RPM and transmitting this rotation to the first and second group of compressors.
The first group of low pressure compressors comprise a first compressor (having two groups of compression stages) of the double flow type receiving via two inputs the gas flow at an input pressure (e.g., of about 1.5-2 bar) and an input temperature (e.g., of about 35-50° C.), and outputting the gas flow at an output pressure (e.g., of about 2.5-4 bar) and an output temperature (e.g., of about 85-100° C.). The output gas is then cooled and input to a second compressor and third compressor. The second and third compressor are grouped in a two-section type compressor.
The second compressor receives an input gas flow at a first input pressure (e.g., 2.5-4 bar) and a first input temperature (e.g., of about 35-45° C.) and outputs a first output flow at a first output pressure (e.g., of about 5.5-7 bar) and a first output temperature (e.g., of about 85-95° C.). This first output flow is then cooled and input to the third compressor as a second flow at a second input pressure (e.g., 5-7 bar) and at a second input temperature (e.g., of about 35-45° C.). The third compressor then outputs a second output flow at a second output pressure (e.g., of about 10-12 bar) and at a second output temperature (e.g., of about 85-95° C.).
The compression of the gas taking place in the first, in the second and in the third compressor (or group of compressor stages) represents respectively the first, the second and the third stage (or phase) of the primary compression.
The output gas is then cooled again and input to the second group of high pressure compressors, comprising a fourth compressor and a fifth compressor. The fourth and fifth compressor are again grouped in a two-section type compressor.
The fourth compressor receives an input gas flow at a third input pressure (e.g., 10-11 bar) and at a third input temperature (e.g., of about 35-45° C.) and outputs a third output flow at a third output pressure (e.g., of about 15-25 bar) and a third output temperature (e.g., of about 85-95° C.). This third output flow is then cooled and input to the fifth compressor as a fourth flow at a fourth input pressure (e.g., 18.5-20 bar) and at a fourth input temperature (e.g., of about 35-50° C.). The fourth compressor finally outputs a fourth output flow at a fourth output pressure (e.g., of about 30-40 bar) and at a fourth output temperature (e.g., of about 85-95° C.).
The compression of the gas taking place in the fourth and in the fifth compressor (or group of compressor stages) represents respectively the first and second stage (or phase) of the secondary compression.
The footprint of the first group of low pressure compressor is relevant due to the presence of the first compressor of the double flow type, and of the second and third compressor of two-section type.
Moreover, the reliability of the entire system depends on the reliability of the gearbox, which is also a costly component.